1. Field of the Invention
This invention relates to a frequency compensation configuration for integrated circuit (IC) controllers using closed loop feedback. In particular, this invention relates to a circuit for summing the difference between two voltages with another variable, such as, for example, a current.
2. Background of the Related Art
In electronic circuit implementations of IC control systems, it is often required to sum a difference between a variable voltage and a reference voltage with another variable such as a current. It is also required to further process the summed result with an integrator that may, in addition to the pole near zero frequency, also have a real zero at a finite frequency, and to output this result as a voltage to be used in other functions.
This function would ordinarily be accomplished with a circuit implementation utilizing an operational amplifier as shown in FIG. 1. Referring to FIG. 1, the circuit includes an amplifier 20 having a gain of A, a voltage source 1, a current source 2 and a reference voltage 4. The voltage source 1 is coupled to a negative input of the amplifier 20 through a resistance 3 and the current source 2 is also coupled to the negative input of the amplifier 20. The reference voltage 4 is coupled to a positive input of the amplifier 20. The output of the amplifier 20 is coupled to the negative input through a feedback connection comprising a resistance 5 and a capacitance 6.
Assuming the input impedance of the amplifier 20 is large enough and initial conditions are ignored, the transfer function in the frequency domain of the circuit shown in FIG. 1 is:
                                                        -                              V                OUT                                                    (                                                                                          V                      1                                        -                                          V                      REF                                        +                                                                  V                        OUT                                            A                                                                            R                    ⁢                                                                                  ⁢                    s                                                  -                                  I                  2                                            )                                =                                                    S                ⁢                                                                  ⁢                                  R                  F                                ⁢                                  C                  F                                            +              1                                      S              ⁢                                                          ⁢                                                C                  F                                ⁡                                  (                                      1                    +                                          1                      A                                                        )                                                                    ,                            (        1        )            where A is the gain of the amplifier 20. If A>>1, then the term 1/A becomes negligible. It is noted that this expression can be extended with additional voltage variable and/or current variable inputs (in addition to V1 and I2).
However, there are difficulties when implementing this circuit in an integrated circuit. For example, the required value CF of the capacitance 6 may be difficult to realize because of the physical size of the capacitance in the integrated circuit. It is also difficult to initialize a desired value of voltage on the capacitance 6 because neither of its terminals is grounded.
Therefore, there is a need for a circuit capable of summing the difference between two voltages with another variable such as a current that overcomes aforementioned difficulties associated with prior art circuit designs.